Yes, you can easily attach computer fans to a car battery. However, a fully charged car battery is actually over 12v and can have up to 15v on it's terminals when being charged (see source). This can easily be sorted by placing a some silicon rectifiers in series to drop the voltage a little which is possibly easier than using dropper resistors (as the diodes act as constant voltage drops) or more complex regulator circuits which are not really needed in this case. Each one will drop the voltage by about 1v, so I'd suggest three 1N4001 in series. They just need to be of a greater current handling than your fan. The band of one should go to the other end of the next. Then, connect the band of the final one to the red (=ve) of the fan. The black of the fan then goes to the -ve of the battery, whilst the other end of the rectifier chain goes to the positive of the battery.
As to how much your fan takes, it will usually be stamped on it. It will be usually between 200-350mA (0.2-0.35A). As car battery is usually about 40Ampere hours (40Ah) so to work out how log you can run your fans for in hours, simply divide 40 by the combined fans current. For example, if each fan takes 0.2A, the total is 0.4A. Therefore your battery will last for 40/0.4=100 hours. however, you should never run a lead acid car battery down to zero, that's asking to shorten its life dramatically or make it totally dead and un-rechargeable.
However, some of the other answers don't make sense and are plain wrong, which shows the danger of putting questions like this on un-moderated sites such as this:
"no u will fry the fans fast u got a lot of amperes there!! a computer fan cant take that kind of amps :/ u can try 4 r 6 D size battery's they should last longer"
The fans will get fried by over-voltage, not the ability of the car battery to provide lots of amps. D size batteries will definitely NOT last longer. A D cell will be usually only be a few Ah, so they will definitely last less. Even the very best will be 10Ah at the absolute most and hence only last a quarter of the time.
ishootbirds2: "car batteries also store a deadly amount of current, enough to kill half a dozen humans, so take extreme caution when hooking anything up to them. make sure you don't make shorts, your wiring is well done." Car batteries can not kill humans as their VOLTAGE is too low.. IF they could, there would be a lot more dead motor mechanics. It's perfectly safe to put your fingers across a car battery. You won't feel a thing and you certainly won't die. however, they are heavy, so drop one on your head, then they can kill.
At least Technobuff knows what he's talking about!
· 1 decade ago